spark-splitter: Add size attribute

Fixes https://github.com/dart-lang/spark/issues/731

TEST=manual. (set size property to the <spark-splitter> from spark_polymer_ui.html.

One problem, the style is explicitly exposed, if this doesn't looks good, I'll try with inner `<div>` tag.
![image](https://f.cloud.github.com/assets/756988/1994954/6cd24616-84fa-11e3-92d1-e9fb9250686c.png)